We have been members for ages, in Canada.
Their own Kirkland brand is consistently high quality. We get their loo rolls and paper towel now mainly because of the quality. Loo roll is soft but effective, and having ulcerative colitis I am a connoisseur!
They have excellent return policies. Never had a problem returning anything. This in itself has made it worthwhile.
We get eggs, laundry and dish detergent, household cleaners etc. which are all well priced. Cereals and spices are cheaper than anywhere else for us. We also get cheese, which we have learned actually freezes well. I keep some flaked Parmesan in the freezer and it tastes great in pasta. We also get good bread.
Their clothing, such as yoga pants, are really good quality and inexpensive. Same with socks and underwear.
It was probably more worth it financially when we had teen boys to feed, but I would miss it if we dropped our membership.
The one big downside here is crowds and parking. Costco is very popular with immigrants from Southeast Asia, who are frugal shoppers and often shop in large family groups. On weekends it is like bumper cars for shopping carts. I understand we have the highest grossing Costco in North America.